High power semiconductor lasers play an important role in a wide variety of applications including direct materials processing and pumping diode pumped solid state lasers and cladding pumped fiber lasers. Achieving the power levels required for these applications is often accomplished by combining the outputs of many lower power laser diodes. As the light output from laser diodes is highly divergent it is often convenient to make use of fiber coupled laser diodes and perform the beam combination with a fiber based beam combiner.
Numerous examples of such beam combiners have been described in the literature. For example, U.S. Pat. No. 5,864,644 to DiGiovanni et al. describes a cladding pumped optical fiber device. The device is based on an architecture that uses a plurality of multimode fiber coupled laser diodes that are each optically coupled to the cladding of a fiber laser. In this geometry, numerous fibers are brought together to a bundled region where they are fused and tapered to a smaller diameter. The bundle is subsequently cleaved and spliced to the cladding of the circular cross section active fiber.
While the fiber based combiner described by DiGiovanni et al. was specific to a cladding pumped device, such combiners can find other applications. Unfortunately, limitations associated with DiGiovanni's combiner and other similarly described devices (referred to herein after as “standard combiners”) have limited their utility in other applications.
One limitation is that the far field pattern generated by standard combiners is sensitive to the routing of the fiber. Small changes in the path of the fiber, as can result from touching or moving the fiber, can result in dramatic changes to the output beam profile. While these changes in far field profile may have little impact in low absorption cladding pumped devices due to the exceptionally long interaction length, they have a severe impact in applications that have reduced interaction lengths, for example, direct materials processing or pumping high absorption laser gain materials such as rare earth doped crystals or highly doped optical fibers.
Another limitation of standard couplers is that the output beam generated by such devices is round. In materials processing applications, there are many applications that benefit from a square or rectangular beam profile. In these applications, a beam is typically scanned along the surface of material that is being treated or otherwise altered. When a circular beam is scanned across the material, the portion of the material that is exposed to the center of the beam experiences a much higher overall dosage of radiation than the portion that is exposed to the edge of the beam. This variation in total dosage leads to an undesirable non-uniformity in the material processing parameters. A coupler is therefore needed that could provide a square or rectangular output beam.